I would go with the cone 5 porcelain. It will be hard and durable. The construction and design of the forms are just as important when it comes to chip resistance, though. Sharp or thin edges will cheap easily. So you may want to redesign your forms to make the disc shaped collars thicker or narrower. Also make sure the bottom edges of the pieces are slightly rounded, not square. Don't feel like your pieces have to match the traditional chess forms.

Just about any clay is suited for these pieces because the nature of the pieces are short, stocky and heavy bottomed. I agree with Neil about your design, avoid thin, narrow necks, pointed, long and unbalanced horse heads. You can also finish the bottoms smoothly so they will glide. A well made cloth surfaced board, in dark velvet or felt can help avoid breakages and make the set appear a bit more elegant.

Porcelain #65 is very sturdy. It is a good idea as Amaco suggests to fire the bisque at the high end of cone 04 (1971°F) for glaze application. At 2205°F, you will get a very good product that is compact and dense and 'rings' on thin pieces when thumped. The Amaco glaze choices are also quite remarkable at cone 5-6.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

Hold it! Clay and glaze must be compatible. Amaco's #25 has a firing range from cone 05 to cone 3; as far as I know Amaco does not make cone 3 glazes. But there are other companies that may make them or you can formulate them or mix them from a recipe. So, if you decide to use Amaco’s #25 low fire clay, use a compatible glaze that at least matures up to cone 3. That also means test, test and test.

#65 Porcelain is considered a midrange clay body and has no ‘range of firing’ it is a cone 5 clay. The glazes you can use mature at cone 5 or less but be sure to bisque fire to cone 04-1971°F. It does make alot of difference for glaze application. Be sure to test.

If using another company’s glazes, test, test and test to see if they are compatible with the clay.

Please if this is not clear, please say so and maybe I can or someone else can explain it better.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

Yes, they are compatible very much so and they are beautiful together. But do a test of the colors you are planning to use to see how they run. Make some test tiles that stand up like a ‘tent’ then you can write the glaze name and number on the insides. Use a dark underglaze pencil (they work better dipped in water) and draw a visible line across an area on the tile and apply your three heavy coats of glaze above that line. Flat tiles will not reveal the fluidity of the float glazes, but they will show the colors and textures of the PC 'Typical Transparent or Opaque Glazes'. I also made small Sake test cups just like Amaco's test vessels however I made them with small troughs and a sprig to catch and watch the flows. I concentrated the 3 heavy coats on the inside of the vessels until I understood the nature of the glazes. I used two or so coats on the outside but feathered up from the foot. I bisque fired to Cone 04-1971°F and glaze fired to Cone 5-2205°F. Be sure to read the labels on the glaze jars some of the glazes say fire 'at Cone 5-6'.

I did fast fire the glaze load because the bisque fire was so high and was only 234 degrees from the mature glaze temperature.

If you do what Amaco suggests you should have success. They even provide a picture to show how to load up your fan brush. Don't be timid about the application. I had no excess running but still be sure to kiln wash your shelves just in case.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

Sorry, I overlooked your concerns about the ware that you have already fired. I would just leave it as it is and take it as a test. You can make your next batch sturdier and fire to clay and glaze maturity. But first test the clay and test the glazes.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

The problem I see is your clay has a firing range of cone 05 to 3, and you are firing to the lower end of this range and having problems with chipping, correct? At the lower end, the clay is probably not vitrified and that means its not as strong as it can be. I dont know the specific products you are talking about but some of the lower fire glazes will fire succussfully at higher temps, so a test would be in order.

If people want unbreakable chess pieces, they can buy ones made from metal.Don't forget the aesthetics of the object. Why would people buy them ? because they are beautiful or because they are rock-hard? People who buy chess pieces made from glass or ceramics know what they buy.By nature ceramic objects are breakable if you mistreat them, no need to cripple your style to make them harder. As you say, they hold up fine.

In all probability the chess pieces will get lost long before they break.Make a few extra pawns.